THIS invention relates to a method of doping particulate semiconductor material. In particular, the invention relates to the doping of semiconductor nanoparticles, but has general applicability to the doping of particles with a wide range of sizes.
Semiconductor nanoparticles, with a characteristic size of a few nanometres up to several hundred nanometres, are a widely studied type of material, in which size effects dominate over properties of the bulk material. In general, depending on the specific material and its application, three different size-related phenomena may change the electronic, optical, thermal and mechanical properties of such nanoparticles:    1. a different structure and composition compared to the known bulk phases;    2. the higher surface to volume ratio of the particles compared to the bulk phase, which causes surface states and processes to dominate; and    3. quantum confinement effects when the size of the object is similar to or smaller than the wavelength and coherence length of a fundamental excitation (electronic state, optical wavelength or phonon excitation).
A specific problem concerns the control of the doping level of either the semiconducting particles, the semiconducting layer, or the composite semiconducting material, as the case may be. In known processes, use has been made of previously doped bulk material which is comminuted to a small particle size. Another possibility is to incorporate dopant atoms into the nanomaterial during bottom-up synthesis of nanoscale clusters. In all cases, even though the doping type (n or p) is normally maintained on the nanoscale, the electrical characteristics of the particles, and compositions thereof, may differ significantly from the bulk prototype, and be difficult to control.
It is an object of the invention to provide an alternative method of doping particulate semiconductor material.